1. Field of the Invention
The present invention relates to a display device that applies a sub-field method to represent a halftone.
2. Description of the Related Art
Current types of display devices are equipped with a plasma display panel (hereinafter, referred to as PDP), or an electroluminescent display panel (hereinafter, ELPD) as a thin flat display panel. In these PDP and ELDP, light-emitting devices, as pixels, are to be in only two states of “light emission” and “no light emission”. In consideration thereof, to derive halftones corresponding to any incoming video signal, a sub-field method is applied to halftone-drives for display panels such as PDPs and ELDPs.
With the sub-field method, an input video signal is converted into N-bit pixel data for every pixel. Based on each of the bit digits of the N bits, a field display period is divided into N sub-fields. The sub-fields are each assigned to the number of light emissions, which corresponds to the respective bit digits of the pixel data. When the logic level of one bit digit in the N bits is “1”, in the sub-field corresponding to the bit digit, the light is emitted for the number of assigned times described above. On the other hand, when the logic level of the bit digit is “0”, in the sub-field corresponding to the bit digit, the light is not emitted. With such a driving method, the number of light emissions is summed up for every sub-field in a field display period. Based on the summed value, the halftone corresponding to an input video signal is represented. Japanese Patent Application Kokai No. 2004-240103 has recently proposed another type of driving method. In the driving method, an input video signal is used as a basis to generate brightness frequency data on a screen basis. The brightness frequency data represents the frequency for each level of brightness. Based on the resulting brightness frequency data, the number of sub-fields is adjusted for every brightness region depending on its frequency. This driving method provides favorable tone representation suiting the characteristics of human sight by assigning the larger number of sub-fields to the brightness segment region of a frequency larger in value.
The problem with such a driving method is that, however, if any high-bright text display such as a news flash, e.g., about an earthquake, is made during image display of television broadcasting, the number of sub-fields to be assigned to the brightness segment regions is abruptly changed. The resulting display makes viewers feel that something is wrong.